1. Field of the Invention
This invention relates to an inspection device and method for detecting the existence of an object to be inspected or an abnormality in the object by use of electromagnetic induction, and more particularly, to an inspection device and method for precisely inspecting the conditions such as internal defects of the objects of various materials using a change in amplitude and/or phase of an electromotive induction current induced electromagnetically by an induction coil placed in an electromagnetic field.
2. Description of the Prior Art
Magnetic flux changes with the existence of an object in an electromagnetic field generated with application of an alternating current, thereby changing the inductance of a coil placed in the electromagnetic field. The inductance of the coil changes proportionally with the dielectric constant, magnetic permeability, size, relative position and other possible inductive factors of the object placed in the electromagnetic field. When some of the inductive factors of the object located in the electromagnetic field are known, the other unknown factors can be calculated from the detected change of the inductance.
A variety of non-destructive inspection devices using such principles of electromagnetic induction are known for recognition of the existence or identification of given objects.
One of the typically known inspection devices of this type is shown in FIG. 1. This inspection device comprises an electromagnetic coil 1 (self-induction coil) which generates an electromagnetic field with an alternating current and is provided in one arm of a bridge circuit 2. In the bridge circuit, a pair of resistors R1 and R2 are equal in impedance to each other. When the coil 1 is excited in the normal state in which no substance exists in the electromagnetic field induced by applying an alternating current from a power source 3 to the coil 1, the inductance of the coil 1 is equal to an adjacent inductor L, and therefore, the bridge circuit 2 on the whole is in equilibrium. In this steadily balanced state, a measuring instrument 4 (usually a galvanometer or sensitive micro-ammeter) connected to diagonal output points P1 and P2 of the bridge circuit 2 generates no output (Vout=0).
However, when an object S to be inspected is placed in the elecltromagnetic field M induced by the coil 1, the self-inductance of the coil 1 changes with the coefficient of induction of the object S, thereby breaking the equilibrium state of the bridge circuit, resulting in a non-equilibrium output voltage Vout across the output points P1 and P2. Generally, the non-equilibrium output from the bridge circuit is amplified by a differential amplifier 4.
By analyzing the change of the output voltage Vout, not only the physical properties and size of the object but also the speed at which the object moves in the magnetic field can be determined. In this case, the bridge circuit is formed in the equilibrium state when a reference body is placed in the magnetic field generated by the inductor L and a normal object is placed near the coil 1.
The aforementioned inspection device is commonly called the "self-induction type" inspection device.
Another type of known inspection device uses mutual induction as shown in FIG. 2. This inspection device 5 comprises an exciting coil 7 (primary coil) excited by a power source 6 to generate an electromagnetic field M and a pair of induction coils 8a and 8b (secondary coil) which acquires the electromagnetic field (magnetic flux) generated by the exciting coil 7 and inducing an electromotive current, and a differential amplifier 9. The induction coils 8a and 8b are wound in opposite directions to each other and are differentially connected in series, so that the electromotive currents induced in the respective induction coils 8a and 8b by the electromagnetic field from the exciting coil 7 cancel each other in the normal equilibrium state. That is to say, in the equilibrium state of the induction coils 8a and 8b, the differential voltage across the output points P1 and P2 becomes zero, i.e. Vout=0.
In general, the mutual induction type inspection device has an inspection path (space d) between the exciting coil 7 and paired induction coils 8a and 8b for allowing a given object S to pass therethrough across the magnetic flux M of the magnetic field generated by exciting coil 7. When passing the object S through the inspection path across the magnetic flux induced by the exciting coil 7, the magnetic flux which reaches the induction coils 8a and 8b undergoes a change in interlinkage. Namely, the paired induction coils 8a and 8b respectively acquire different interlinkage numbers of the magnetic flux, to thereby break the balanced state of the induction coils 8a and 8b (nonequilibrium state), As a result, a differential voltage Vout is generated from the differential amplifier 9 . Thus, it is possible to recognize the physical properties and size of the object S or to detect defects such as a crack and pin hole in the object S.
In the prior art inspection devices noted above, since the nonequilibrium state in electromagnetic induction is determined using the differential voltage derived from the bridge circuit 2 or the series connected induction coils 8a and 8b, the change in induced electromotive current which is caused by passing the object across the magnetic flux must be measured with a notably high accuracy in order to increase the measurement accuracy.
In the self-induction inspection device illustrated in FIG. 1, however, because the rate of change in self-induction (difference between the base inductance and the inductance undergoing a change) is very small, it has been substantially impossible or difficult to precisely detect such a small change in inductance. Thus, the conventional inspection device of the self-induction type has a low sensitivity and cannot be applied to inspect a given object having a low rate of change in inductance and nonmetallic objects such as synthetic resin.
On the other hand, the mutual induction inspection device of FIG. 2 has the inspection path defined between the exciting coil 7 (primary coil) and paired induction coils 8a and 8b (secondary coil). The induction efficiency is in inverse proportion to the dimension of the inspection path (space d between the exciting coil and the paired induction coils). This device is disadvantageous in that the inspection path is limited in dimension from the standpoint of performance and adds to the total system size and prevents a large object to be inspected from passing therethrough.
Even if the inspection path is made wide for permitting such a large object to pass therethrough, the inspection accuracy is decreased proportionally and a slight change in induction cannot be detected.
Moreover, the inspection device of the mutual induction type inevitably has a fatal disadvantage in that, when the object S approaches one of the paired induct/on coils (coil 8a in FIG. 2), not only the coil 8a but also the coil 8b is affected by the object S to cause the coil 8b to change the inductance of the coil 8b. Though either of the induction coils should have, as a reference inductor, a fixed inductance relative to the other coil close to the object S, both the coils change in inductance even when the object S approaches one of the induction coils. Namely, the induction of the induction coil remote from the object S changes in a complicated manner with the relative position of the object S to the induction coils. Therefore, a change in inductance of one of the induction coils cannot be determined precisely, resulting in a conspicuous decrease in measuring accuracy.
As noted above, the conventional inspection devices using electromagnetic induction are restricted in the size of the object to be inspected and inevitably lead to noticeable measurement errors. Though the inspection device of this type may use an exciting current having adequate frequency with which electromagnetic induction changes markedly to increase the detecting sensitivity, it has a common disadvantage of being so limited in their range of applications as to be of no practical utility. Further, the conventional inspection device suffers a serious disadvantage such that occasionally, a change of the electromagnetic induction, which is caused by a defect in an object, is not noticeably different from that by a nondefective object. Under such circumstances, the defective object would be Judged to be good in error. Such inexpediency is possibly brought about even when the measuring sensitivity is increased. For example, when a weld portion (nugget) formed by spot welding in a metallic object is subjected to a non-destructive inspection using a change of electromagnetic induction, the electromagnetic induction detected from a normal nondefective object by the conventional inspection device using electromagnetic induction is not always fixed.
Also, not infrequently there are times when the defects such as a crack and incomplete fusion in the weld portion can hardly be detected in the form of a change of inductance according to the locations of the defects even if they are sufficient for being detected practically. In this case, disadvantageously, the defective object is possibly taken for a nondefective object.